(Meth)acrylic polymers are known to exhibit effects as flocculation and thickening properties and are in use, in the field of medicines, as additives and hydrophilic ointment bases for the purpose of improving the adhesiveness and water retention of fomentations and cataplasms. In the field of paints, these polymers are used as carpet compound thickeners, and as paint thickeners or tackifiers or adhesiveness improvers. In production processes, they are applied for many different applications as red mud settling agents in the production of alumina, and as flocculants for saline water purification in the soda industry. In the field of civil engineering and construction, these polymers are used as dug soil treating agents, dredged soil treating agents and mud modifiers. For example, dug soil treating agents containing such (meth)acrylic polymers are used to solidify hydrous soil resulting from drilling engineering for reutilization of the same. Furthermore, they are used as moisture absorbents, desiccants, surface modifiers, and various thickening agents in other fields of industry in general.
Since (meth)acrylic polymers can be used in such useful fields of application, various investigations have been made to improve their performance characteristics.
Meanwhile, investigations have been made also on the method of producing (meth)acrylic polymers by photopolymerization. Concerning such technology, Japanese Kokai Publication Sho-48-58079 (Japanese Kokoku Publication Sho-56-15808) discloses a method of obtaining low-molecular-weight acrylic polymers and copolymers by photopolymerization. The above-cited Japanese Kokai Publication Sho-48-58079 describes an example in which methanol is added as a chain transfer agent and the photopolymerization is carried out at a concentration, on the occasion of polymerization, of the monomers used of about 60% by weight to about 80% by weight. However, the main component of the monomer composition used is acrylamide, and acrylic acid, which is one of (meth)acrylic acid monomers, is used only in an amount of several percent; this production technology is thus concerned with water-soluble polymers different from the water-soluble polymers which the present invention is to provide. In addition, the polymers disclosed in the above-cited publication are not polymers having sufficient physical properties desired for use as dug soil treating agents or cataplasm additives.
Japanese Kokai Publication Sho-61-155405 (Japanese Kokoku Publication Hei-5-53804), Japanese Kokai Publication Sho-61-155406 (Japanese Kokai Publication Hei-5-32410), Japanese Kokai Publication Hei-10-231309 and Japanese Kokai Publication Hei-11-228609 each discloses a method of producing water-soluble polymers by photopolymerization. The above-cited Japanese Kokai Publication Hei-10-231309, for instance, describes that when the amount of the photopolymerization initiator is reduced, the generation of heat and abrupt progress of reaction can be prevented, with the result that polymer gels having a thickness of at least 30 mm can be produced. The monomer compositions used here mainly comprise acrylamide, with a slight amount of acrylic acid, as similar to the method in Japanese Kokai Publication Sho-48-58079. Thus, the polymers disclosed there differ from the water-soluble polymers which the present invention is to provide; these polymers are not polymers having sufficient physical properties desired for use as dug soil treating agents or cataplasm additives. Japanese Kokai Publication Hei-10-231308, the applicant of which is the same as that of Japanese Kokai Publication Hei-10-231309, discloses a technology of reducing the heat generation during polymerization by using an acylphosphine oxide as a photopolymerization initiator. The main component subjected to polymerization according to Japanese Kokai Publication Hei-10-231308 is acrylamide and the amount of acrylic acid is small, hence the resulting polymers are different from the water-soluble polymers to be provided in the present invention. These publications do not disclose the use of a chain transfer agent.
The above-cited Japanese Kokai Publication Hei-11-228609 describes, as an efficient cooling method during polymerization, a technology of cooling the aqueous solution of monomer by covering the same with an optically transparent film. Although it describes the use of a chain transfer agent and the mode in which a monomer concentration during photopolymerization is 25% by weight to 80% by weight, the main modes of practice are directed to the polymerization of acrylamide and dimethylaminoethyl acrylate; this technology is not one concerning the mode of polymerization of monomer compositions in which acrylic acid is a main component. Although there is described the mode of practice in which acrylic acid is used in part, no chain transfer agent is used there and the concentration in the step of polymerization is as low as 35%; there is no disclosure about the physical properties desired of the acrylic acid water-soluble polymers which the present invention is to provide. Further, as for the acrylamide polymers as polymers to be used in soil treating agents or cataplasm additives, the toxicity of acrylamide possibly remaining after polymerization is a problem and there is a tendency toward restriction of the use of those polymers. The water-soluble polymer technologies that are the subject matters in the publications cited above have a problem in this respect.
Japanese Kokai Publication Sho-61-155405 and Japanese Kokai Publication Sho-61-155406 also describe a method of photopolymerization on a belt using likewise an acrylamide monomer. As for the monomer concentration in the reaction mixture to be subjected to polymerization, Japanese Kokai Publication Sho-61-155405 describes a concentration range of 20 to 45% by weight. However, like in the other publications discussed above, each technology is directed to the production of acrylamide polymers, namely polymers differing from the water-soluble polymers of the present invention which are obtained by polymerization of a monomer composition comprising acrylic acid as the main component.
Further, Japanese Kokai publication Sho-62-156102 (corresponding to U.S. Pat. No. 4,857,610 and U.S. Pat. No. 4,893,999) discloses a technology of polymerizing a water-soluble monomer(s), typically (meth)acrylic acid, acrylamide, acrylamidemethylpropanesulfonic acid and/or dimethylaminomethyl acrylate, by feeding the same on a belt under photoirradiation. This publication describes a mode of continuous polymerization using a belt polymerization apparatus having a specific form and structure and, as regards the water-soluble monomer concentration in the reaction mixture, the specification thereof describes a range of 16 to 60% by weight. No chain transfer agent is used, however. Thus, the physical properties of the resulting water-soluble polymers do not exceed the levels conventional in the art. The modes of polymerization as disclosed in the examples do not include any mode of polymerization using, in particular, a (meth)acrylic acid monomer as the main monomer component and setting the monomer concentration at a high level. Thus, Japanese Kokai Publication Sho-62-156102 does not describe any specific conditions concerning the production of those water-soluble (meth)acrylic polymers improved in physical properties which the present invention is to provide. The above publication contains no description about the specific uses of said water-soluble polymers as dug soil treating agents and cataplasm additives.
In Japanese Kokai Publication Sho-52-47084, there is disclosed a method of radical polymerization or copolymerization of a water-soluble monomer or monomer mixture either alone or in admixture with an unpolymerizable inert substance under UV irradiation. In Japanese Kokai Publication Sho-55-50002, there is disclosed a method of radical polymerization or radical copolymerization in which the polymerization process is adjusted by increasing the intensity of ultraviolet irradiation with the lapse of time. However, the water-soluble polymers described in the above-cited publications are acrylamide polymers, which differ from the water-soluble polymers to be provided by the present invention. Furthermore, Japanese Kokai Publication Sho-52-126494 (Japanese Kokoku Publication Sho-55-12445) discloses a method of producing high-molecular-weight water-soluble acrylic polymers which comprises continuously feeding an aqueous solution of an acrylic monomer or monomers onto a support to deposit the solution as a thin layer and irradiating this liquid thin layer with actinic radiation at a specific mean output. While it is described on page 4 of this specification that the monomer concentration in the polymerization is preferably 20 to 60% by weight, the water-soluble polymers disclosed in the examples are exclusively in the fully neutralized form, namely in the form resulting from complete neutralization of 100 mole percent of the acid groups which each water-soluble polymer has. Therefore, there is no specific description about the partly neutralized or unneutralized, water-soluble (meth)acrylic polymers which the present invention is to provide. The polymers described in this publication are acrylamide polymers differing from the polymers which the present invention is to provide.
Furthermore, according to these technologies, the intensity of exposure light is increased after the initiation of polymerization and, therefore, there is room for contrivance for adequately adjusting the light intensity so that polymers capable of fully exhibiting their useful effects in various fields of application may be produced while inhibiting an uncontrolled polymerization due to an anomalous reaction from occurring and thus increasing the safety of the process.
As regards the technology of dug soil treating agents using (meth)acrylic polymers, Japanese Kokai Publication Sho-64-81886 (Japanese Kokoku Publication Hei-3-2478) discloses a hydrous soil solidifying agent comprising a (meth)acrylamide (co)polymer and gypsum, and Japanese Kokai Publication Hei-4-345685 (Japanese Patent Publication No. 2529785) discloses a modifier for hydrous soil which comprises a carboxyl group-containing, water-soluble polymer, such as a sodium acrylate-acrylamide copolymer, and gypsum. However, the use of the acrylamide polymers according to these technologies is much restricted in view of the toxicity of the residual monomer (acrylamide). Japanese Kokoku Publication Hei-6-31514 (Japanese Kokai Publication Sho-63-44097) discloses a method of treating dug soil using a specific, water-soluble synthetic polymer substance. The water-soluble polymer substance is high in safety but a relatively large amount thereof is required for solidifying dug soil, hence an improvement in this respect is desired. Furthermore, Japanese Kokai Publication Hei-9-176638 discloses a modifier for hydrous soil which comprises an aqueous solution of polyacrylic acid and/or a salt thereof. Although this modifier in which polyacrylic acid is used is evaluated relatively highly, there is room for contrivance for still more improving its performance characteristics when it is used at low addition amounts.
More specifically, the above-cited Japanese Kokai Publication Hei-9-176638 discloses, in Example 1, a species of polyacrylic acid which has a weight average molecular weight of 800,000 with a neutralization degree of 0%. The above document, however, does not describe by which the method of production the polyacrylic acid was prepared nor describe the insoluble matter content or other data. Further, the water-soluble polymers disclosed in Example 1 and Example 5 in the above publication are not so high in soil treating property; this is presumably due to their low molecular weights. The above-cited Japanese Kokai Publication Hei-9-176638 does not contain any description at all about the fact that when the monomer concentration is increased and a chain transfer agent is used in the production of (meth)acrylic acid based water-soluble polymers by photopolymerization, the intrinsic viscosity (or weight average inertial radius) can be increased, as well as the insoluble matter content can be reduced, and that the (meth)acrylic acid based water-soluble polymers obtained by such method of production can exhibit significant water treatment performance characteristics.
As for the technology of cataplasm additives, completely neutralized forms of polymers, such as poly(sodium acrylate), have so far been used much. However, cataplasms obtained by using such a completely neutralized polymer are low in adhesiveness or tackiness, so that there arise such problems as falling or turning up thereof from the sites of application. To overcome these problems, Japanese Kokai Publication Sho-61-260014, for instance, proposes the use of a partially neutralized polyacrylic acid salt. Japanese Kokai Publication Sho-62-63512 (Japanese Kokoku Publication Hei-6-6533) proposes the combined use of poly(sodium acrylate) of the completely neutralized form and unneutralized polyacrylic acid. Furthermore, Japanese Kokai Publication Hei-8-53346 proposes the use of unneutralized-form polyacrylic acid as the additive. The additives according to these technologies are evaluated relatively highly from the adhesiveness viewpoint but the shape retentivity when they are used is not sufficient in preparing cataplasms; there is room for improvement in this respect. Thus, there is room for contrivance for allowing (meth)acrylic polymers used as cataplasm additives to provide satisfactorily high levels of adhesiveness and shape retentivity. The term “shape retentivity” as used herein refers to a physical property indicating the elasticity of the base of a cataplasm preparation and thus, for example, refers to a physical property indicating the degree of easiness of returning of an indentation made on the cataplasm base surface with a finger or the like to its original shape.
In Japanese Kokai Publication 2000-212222, there is disclosed a partially neutralized (meth)acrylic acid polymer in which an intrinsic viscosity at 30° C. and a insoluble matter content in deionized water (an ion-exchange water-insoluble content) are specified. Such a (meth)acrylic polymer can exhibits effects as those flocculant and thickener which can be used as a dug soil treating agent and a cataplasm additive. However, the method of polymerization disclosed therein is a thermal polymerization method. There is no description about photopolymerization at all. In particular, nothing is taught about the photopolymerization method for producing (meth)acrylic acid based water-soluble polymers in which the polymerization is carried out using a reaction mixture containing a (meth)acrylic acid monomer(s) at a specified or higher concentration in the presence of a chain transfer agent. The water-soluble polymers disclosed in this publication have a neutralization degree (content of neutralized groups contained in the water-soluble polymer as defined in the present application) of 20 to 95 mole percent and are especially low in neutralization degree. There is no specific description or suggestion about those acid form water-soluble polymers which have a low neutralization degree, specifically a neutralization degree of less than 20 mole percent.
It is supposed that when a (meth)acrylic acid based water-soluble polymer with a low neutralization degree and with a high molecular weight is produced by the thermal polymerization method described in the publication cited above, the gel-like (meth)acrylic acid based water-soluble polymer will adhere to the polymerization apparatus and various other apparatus such as a gel disintegrator. Thus, there is room for contrivance to produce the same in more stabilized manner in a commercial scale. Further problems are anticipated, for example the residual monomer content of the gel-like polymer obtained will become increased, and the insoluble matter content will increase since the polymer is in an easily crosslinkable condition. Thus, there is room for contrivance for making it possible to produce water-soluble polymers with a low neutralization degree in a stable manner on a commercial scale applying said method of polymerization.
Those (meth)acrylic acid based water-soluble polymers having a neutralization degree of 20 to 95 mole percent are lower in adhesiveness as compared with the (meth)acrylic acid based water-soluble polymers with a neutralization degree of less than 20 mole percent, however, for producing water-soluble polymers with a higher molecular weight stably on a commercial scale, the thermal polymerization method described in the above-cited Japanese Kokai Publication 2000-212222 has room for contrivance.